With ever-increasing integration density on electronic modules, the number of connections of electronic components is constantly rising. To address this trend installation and contacting methods have been developed by which the components are contacted using balls of solder on the underside of the components with connecting surfaces on a substrate to be equipped. These types of component are typically known as Ball-Grid-Arrays (BGA) or Flip-Chips. To guarantee reliable contacting the connections must be inspected precisely before component placement, since faulty connections that lead to a bad electrical contact between component and the connecting surfaces can no longer be detected.
To enable electronic modules to be manufactured at low cost and with high quality a large number of demands are imposed on modern inspection systems for electrical components. The inspection system must thus be in a position to determine the parameters of the inspection object, such as typically its dimensions, the coplanarity of the electrical connections or the pitch of the connections. Furthermore it should be possible to carry out the inspection within the shortest possible time, and at low cost, as a non-contact process. These strict demands for the measurement of three-dimensional surfaces can as a rule only be fulfilled by an optical method for measuring surface profiles. Optical methods of inspection known in this context are delay methods, triangulation methods and confocal methods.
The delay methods in which the distance between sensor and the surface to be measured are determined from the delay of a pulse of light reflected back from the surface, also include in this con-text what are known as the interferometric method. This can achieve higher spatial resolutions by overlaying coherent beams of light. The interference signal is created by a mechanical movement of an optical element of an interferometer or. a modification of the optical path length within an Interferometer. In this case, especially for a surface image recording of a surface to be measured relatively long measurement times are required.
The Triangulation method also includes all methods for which the direction of illumination or projection direction deviates from the direction of observation. This also includes methods that operate by way of structured illumination (e.g. moiré method) since the deformation of the pattern projected on the surface to be recorded from which deformation the height position of individual surface points is calculated, can only be observed from a specific angle of triangulation. The measurement of three-dimensional surface profiles by way of structured illumination ideally requires isotropically scattering object surfaces, since anisotropically scattering surfaces, i.e. at least slightly reflective surfaces cannot reflect the deformation of the structured illumination through the three-dimensional surface because of a mirror lens effect. Since the reliability of the measurement results is very strongly dependent on the reflection behavior of the surface to be measured, solder ball inspection using structured illumination is generally impossible or extremely difficult to implement.
The triangulation method also includes so-called triangulation methods in which the surface to be measured is scanned with a laser beam and point of incidence of the laser beam is recorded by a camera. Fast deflection units, such as rotating polygon mirrors or galvanometer scanners are used here for defined deflection of the laser beam. Alternatively a relative movement between the object and the laser beam can be created by a movement of the object to be measured. Measurements by laser triangulation have the disadvantage that a number of surface points cannot be scanned separately but only in sequence one after the other, so that the resulting test times are corresponding long.
The outstanding features of confocal optical methods for calibrating three-dimensional surfaces are high resolution and high levels of robustness with regard to scattered light created by secondary reflections. Confocal optical methods have the further advantage that surface measurement can be undertaken coaxially so that shadowing problems by illumination light falling on the surface at an angle or by observation at an angle to the surface do not occur. Confocal microscopy, which has been known for some time, thus represents a very precise but slow method for three-dimensional surface measurement Conventional confocal displacement sensors also have the disadvantage that a periodic relative movement between the sensor and the surface to be measured is required, so that as result of the of the mass inertia of the masses to be moved, the scanning rate is additionally restricted.
A modified confocal sensor for three-dimensional surface measurement is known from EP 835423 B1 which allows a rapid measurement of the surface through a rapid shift of focus is effected by a mechanically moved retro-reflector using a linear arrangement of plurality of laser beams The image recording is thus comparable with a line camera, by which by a movement of the object to be measured and or the camera in a direction at right angles to the camera line endless images can be recorded in principle. For this reason the modified confocal sensor is also suitable for measurement of larger objects such as wafers or substrates. Since the width of the image is determined by the length of the scanned line, larger image areas must be measured by meander-type scanning of the surface. The disadvantage of the modified confocal sensor is that the required focus shift is generated by a movement of the retro-reflector, so that although a smaller mass has to be moved compared with conventional confocal displacement sensors, the mass inertia of the moved retro-reflector still limits the scanning rate.